Exhaust aftertreatment systems are used to receive and treat exhaust gas generated by IC engines. Conventional exhaust gas aftertreatment systems include any of several different components to reduce the levels of harmful exhaust emissions present in exhaust gas. For example, certain exhaust aftertreatment systems for diesel-powered IC engines include a selective catalytic reduction (SCR) catalyst to convert NOx (NO and NO2 in some fraction) into harmless nitrogen gas (N2) and water vapor (H2O) in the presence of ammonia (NH3). Generally in such conventional aftertreatment systems, an exhaust reductant, (e.g., a diesel exhaust fluid such as urea) is injected into the aftertreatment system to provide a source of ammonia, and mixed with the exhaust gas to partially reduce the NOx gases. The reduction byproducts of the exhaust gas are then fluidically communicated to the catalyst included in the SCR aftertreatment system to decompose substantially all of the NOx gases into relatively harmless byproducts which are expelled out of such conventional SCR aftertreatment systems.
An exhaust reductant is generally inserted into SCR system as the source of ammonia to facilitate the reduction of constituents of the exhaust gas (e.g., a diesel exhaust gas). The exhaust reductant is stored in a reductant storage tank and communicated to the SCR system. The reductant generally includes an aqueous solution such as an aqueous urea solution. A reductant physical level sensor is included in the reductant storage tank which can include, for example an ultrasonic reductant level sensor. Under certain conditions, the reductant physical level sensor may provide unreliable readings and/or a false low level warning of the reductant in the storage tank.
For example, in freezing or sub-zero environmental conditions, the reductant in the storage tank can freeze. A heater is generally provided in the reductant storage tank to thaw the reductant. However, in conventional reductant storage tanks, the heater is positioned at a location in the reductant storage tank, for example at a base of the reductant storage tank so that the reductant near the base of the reductant storage tank melts and becomes a liquid first before a bulk of the reductant is thawed. In other words, the reductant storage tank can be fully or substantially filled with the reductant but only a small portion of the reductant is in liquid phase. This situation can persist for some time before all the reductant thaws and melts.
The ultrasonic waves generated by the physical level sensor are only reflected by a portion of the reductant which is liquid while a frozen and still unthawed portion of the reductant does not reflect the ultrasonic waves. In such circumstances, the physical level sensor falsely indicates a level of the liquid portion of the reductant as the total level of the reductant in the reductant storage tank at any given point in time. If this level is below a threshold level or critical level, a reductant low level indication is erroneously indicated to the user.